My Sansa C250 has worked great for about two

Sandisk about this problem songs not being recognized on the computer but still on the player:
http://kb.sandisk.com/app/answers/detail/a_id/447
Use this links as main help thread.

The standard connection type of C200 series is MTP = Media Transfer Protocol. In this mode you can see the connected device as a portable player.

Songs that are put on the player in UMS = Universal Mass Storage mode (also called MSC = Mass Storage Class), seeing the device as a removable disk can only be seen in the players' UMS mode and vice versa.

Check the setting of the USB mode on the display of the C250:

Main Menu
> Settings / USB / Autodetect or MSC mode

Autodetect = MTP mode: computer sees the player Sansa C250.
MSC mode: computer sees a removable disk.

Now 2 things could have happened:

1. Music is put on the player in MSC mode and now the player is
connected in MTP mode or vice versa.

2. There are bad sectors or corrupted files are transferred on the players' removable disk.

• In case of bad sectors a recovery is possible: in MSC mode perform check disk as told in the link above.

• In case of corrupted files a format of the device is needed. First a backup of all data has to be made. Next perform format. This can be done in 2 ways:

Using disk management:
http://kb.sandisk.com/app/answers/detail/a_id/107
Using Windows Media Player:
http://kb.sandisk.com/app/answers/detail/a_id/444

Option 2. having problems with the disk or data on the players' disk can also be solved by updating the firmware.
For firmware update see link:
http://kb.sandisk.com/app/answers/detail/a_id/315
Remember: before updating backup the data from player to the computer.

When more help is needed just post a comment.